Refinery fractionator wash beds are monitored for fouling/coking of the wash bed on a regular basis. The density of the fouling/coking in the wash beds increase over time, which allows for a prediction of the tower's ability to run efficiently over time.
When the fouling/coking of the wash bed becomes a serious impediment to operating efficiency, the entire process unit may be taken off-stream for an extended period for renewal of the wash beds. This is commonly referred to as a turn-around (TAR).
During a TAR cycle, after the vacuum heater trips and the wash bed temperature drops below 400° F., a density scans show an increase in the slope of the fouling/coking in the wash bed. It has been observed that a thermoset polymer is forming in the wash bed during operations. A thermoset polymer is a petrochemical in a soft-solid or viscous state that changes irreversibly when cured into an infusible, insoluble polymer network. Once the thermoset is cured, then the polymer can only be removed by physically changing the packing in the wash bed during a TAR.